In recent years, many varieties of electronic apparatuses have come into use in homes and the like, and their design for protection against fire has assumed great social importance. Similarly, the industrial apparatuses such as electronic computer, telephone switching systems and the like have become larger year after year, and, accordingly, if a fire breaks out in such an apparatus, the whole of the valuable system could be greatly damaged.
Fusible resistors, which are generally utilized for protection against fire in the types of apparatus as described above, function as ordinary resistors for a normal current flow, but if a circuit should carry any abnormal excess current, a predetermined portion of the resistor in that circuit would be fused to open the circuit, whereby the apparatus is protected against overheating and possible fire.
A resistor usable in the manner described above should satisfy the following requirements:
1. It should function with a reliability equal to that of common resistors, and without danger of causing faulty operation of the apparatus with which it is used;
2. When a current in excess of a predetermined amount flows, it will be fused as quickly and surely as possible to open the circuit; 3. The fusible resistor, when being fused, will not produce an arcing discharge which itself might cause a combustion and produce smoke so as to cause a secondary fire; and
4. The operating conditions at which fusion occurs, i.e. amount of current and duration thereof, are as stable and capable of being accurately determined as possible.